Methods of coloring of metals

 Colouring of metals

.methods of colouring by heat

.different methods of colouring metals

. method of blackening /black oil finishing of metal surfaces

Machined or filled ferrous metal  surfaces can be colored by the following methods.

Heating and cooling

Application of chemicals

Colouring by heating

Attractive colours can be given to metals by  heating. In this process, the metal surface to be treated is thoroughly polished.

The metal is then placed in a heat treatment oven or flame heated.

During the heating of the metal, watch the colours as they appear.

When the desired colour is seen, remove and quench suddenly in water or oil.

Rub gently with steel wool.

Black oil finish

For blackening the surface of a ferrous metal, first clean the surface thoroughly, and apply a coating of thick lubricating oil.

Then heat the metal to 150° to 175° c for about 5 to 8 minutes.

This will give a black and rust preventing coating on ferrous metal surfaces.

Chemical colouring

Polished surfaces of iron and steel can be given the following colours by the application of certain chemicals.

Colour and process

BLUE BLACK 

Process- Mix 16 parts of saltpetre and two parts of oxides of manganese. Heat the mixture to 400° c and dip the cleaned work in it. Keep the workpiece in the mixture and allow it to cool.

BROWN 

Proces- Dip the workpiece in ammonia and dry in open air. Dip the work in hydrochloric acid and dry in warm place. Dip the workpiece in gallic acid and remove it when the desired colour is obtained.

BRONZE

Heat the workpiece slightly and paint a paste of antimony chloride. Watch for the desire shade to appear on the workpiece. Then wipe it dry.

CAUTION

Use hand gloves and goggles when working with chemicals.

Steel ( Plain carbon steel)

Steel is fundamentally an alloy of iron and carbon with the carbon content varying up to 1.5%. The carbon present is in a combained state.

Plain carbon steels are classified according to their carbon countent.

CLASSIFICATION AND CONTENT OF PLAIN CARBON STEEL

DEAD MILD STEEL

The percentage of carbon is 0.1 to 1.25%.

They high ductile. Used for making wire rods, thin sheets & solid drawn tubes.

MILD STEEL

The percentage of carbon is 0.15 to. 3%.

They relatively soft and ductile. Used for general workshop purposes,boiler plates, bridge work, structural sections and drop forgings.

MEDIUM CARBON STEEL

The percentage of carbon is 0.3 to 0.5%.

Used for making axles, drop forgings, high tensile tubes, wires and agricultural tool.

Medium carbon steel is carbon percentage different 0.5 to 0.7%.

This medium carbon steel is used for making steel,locomotive tyres, large forging dies, wire ropes, hammers and snaps for riveters

HIGH CARBON STEEL

High carbon steel is percentage of carbon 0.7 to 0.9%  is used  for making spring, small forging dies, shear blades and wood chisels.

High carbon steel percentage of carbon is 0.9 to 1.1%. It is used for making cold chisels, press dies, punches, wook working tools, axes etc.

High carbon steel percentage of carbon 1.1% to 1.4% is used for making hand files, drills, gauges, metal cutting tools,razors.

Principle of metal cutting

.different angles formed at the cutting edge of tools

. the important of clearance and rake angles in metal cutting

Metal cutting

Knowledge about the principal for metal cutting relating to the various tool angles will assist in carrying out of metal cutting work efficiently.

Workpiece are shaped and brought to the required dimension by the use of cutting tools. While cutting materials the following four important elements interact.

Cutting tool

Tools holding or grinding device

Work holding devices, and work

Although the metal cutting process involves the combination of the above four elements, everything begins with a cutting tool and its impact at the time of contacting the workpiece.

The common metal cutting processes are carried out either by the  use of a single point or multi point cutting tools.

Cutting edge are taken as the factors for determining the tool life, surface finish, and the force required for cutting.

The important angles of the cutting edge are

Clearance angle

Rake angle

Wedge angle or tool angle

Clearance angle

This is the clearance or relief provided behind the cutting edge.

If the clearance angle is not provided the tool will rub against the work surface, and will impose a lot of strain before the metal chips are sheared from the work.

The clearance angle improves these conditions and prevents tool rubbing, thus reducing the cutting force.

The clearance angle depends on the of surface being machined.

Normal clearance angles for different surface are

External cylindrical surface    -5° to 7°

Flat surface                                  -6° to 8°

Internal cylindrical surface      -8° to 10°

Smaller clearance angles tend to rub against the surface while excessive clearance causes chatter, and there  is a tendency for the tool to dig in. This will also result in increased tool angle thereby reducing the tool strength.

Rake angle

This is the angle given in the front of the leading edge of the tool.

This reduces the pressure of the chip on the tool face, and thereby reduces the cutting force ;also, it affects the formation of chips and the surface finish.

Positive rake angles produce better finish at low speeds.

Increased rake angle increases the cutting efficiency but decreases the tool wedge angle, and thereby decreases the strength of the tool. Low strength and ductile materials can be cut with increased efficiency with tools having high rake angles.High strength materials are cut with tools having high rake angles. Low strength materials are cut with tools having low positive rake angles.

Zero and negative rake angle tools are stronger and have a long tool life.

Negative rake tools produce poor finish at low cutting speeds but give good  finish at higher cutting speeds

Negative rake tools need more power to operate than the positive rake tools.

Rake angle of different metals

Cast iron 0°

Ductile brass 14°

Free cutting brass 0°

Tin bronze 8°

Aluminium alloys 30°

Mild steel 25°

Medium carbon steel 25°

High carbon steel 12°

Teflon 0°

The wedge or tool angle is affected by the amount of clearance and rake angle. More clearance and rake angle will decrease the tool wedge angle thereby reducing the strength of the cutting edge.

The rigidity of tool holders and holding devices contributes to efficient cutting.

The design of the holder depends on the type and shape of the tool and its intended function.

The work holder used depends on the shape of the work and the type of force acting during cutting.

Types of metal cutting saws

Different types of cutting saws

Advantages of a horizontal band saw

Different types of cutting saws

Metal cutting saws of different types are used in industries. The most commonly used are the

Power saw

Horizontal band saw

circular saw

Contour saw

Power saw

This is the most commonly used metal cutting saw.

Horizontal band saw

This has a saw frame on which a motor is  fitted. There are two pulley wheeels on which an endless band saw passes.

Speed variation is obtained through the stepped pulleys on the motor.

The roller guide brackets provides the rigidity for the blade in the cutting area and also prevent wandering of the blade while cutting.

The blade tension is maintained by using the adjusting handle, provided for this purpose.

A vice is provided for holding the metal stocks. The vice is adjustable for angular cutting.

The machine has the advantage of continuous cutting ability, and is much faster than a power saw. It may be noted that a power saw cuts only in every alternate stroke.

Circular saw

This type of cutting machine is used  when cutting materials have a large cross section. The circular saw has a continuous cutting action and is economical in production work where heavy section metals are used.

Contour saw

In this a metal band saw blade is used, and the contour saw has a continuous cutting motion.

These machines are very much used for cutting metals to different profiles.

Different speeds can be obtained while cutting, with the help of variable speed pulleys.

For repairing broken countour saw blades, these machines are fitted with a  shear for trimming the blade ends, a but welding machine for joining the ends and the small grinder to finish the welded joint.

The table can be tilted t any angle for angular cutting. The blades passes through a guide which prevents the blades from wandering and keep it rigidity.

These machines are widely used for tool room work, and not as a machine for cutting raw material stock.

Precautions while machine sawing

In order to work safely and efficiently, certain precautions are to be obserened.

While taking measurements of the work for setting,always stop machine.

Projecting ends of the work should be well guarded, so that safety may be provided to others.

Ensure that the work does not protrude into the gangways.

When sawing thin pieces, hold the material flat in the vice to prevent the saw teeth from breaking.

Ensure a cutting fluid is used always.

Avoid giving excessive cutting pressure, because this can cause breakage to the blade, and cut work out of square.

When several pieces of the same length are to be cut, use a stop gauge

When holding short workpiece in a vice, be sure to place a short piece of the same thickness in the opposit end. This will prevent the vice from twisting when it is tight ened

POWER HACKSAW

Features of power hacksaw

features of a power hacksaw

. correct blade of different types

. features of power hacksaw blade

Cut-off saws are used to cut metal stock roughly to the required length. 🤺 commonly used to cut off saw in small scale industries is a POWER SAW.

Power hacksaw

Features

The power saw works like a handsaw, and has an arrangement for cutting during the movement in one direction and releasing pressure on the non-cutting stroke. The rotary motion of the motor is converted into linear motion by a crank mechanism.

The required cutting pressure is obtained hydraulically or by an adjustable weight.

During the non-cutting motion the blade will be lifted away from the work.

A clamping device /vice holds the work firmly.

Power hacksaw blades

The saw blades are selected, depending on the machine and the type of work on hand. The blades are made of H. S. Steel and are fully hardened.

For different materials, blades of different pitches are used (number of teeth per 25mm length ).

As a general rule, the softer the material, the lesser  is the number of teeth, per length of 25mm.

Teeth with a large pitch can accommodate large chips.

Blades are available with varying coarseness i.e between 4 to 14 teeth per 25 mm length.

Coarse pitch blades are also used while cutting large sections of stock, as this will help in greater chip clearance  and increased penetration.

For cutting hard material, and thin material, a 14 pitch blade is recommended.

For general purpose sawing, a 10 pitch blade will be useful.

While selecting blades, make  sure atleast two teeth of the blade will be in contact with the work at all times.

What will happen if less than two teeth are  in contact with the work?

The work can be caught in the tooth gullet, and cause breakage to the blade.

Blade clearance

In order to avoid jamming of the teeth and to provide for chip clearance, the teeth of the saw bladers are offset or waved.

SPECIFICTION OF POWER HACKSAW BLADES

while specifying power hacksaw blades, it is necessary to state

-the length (distance between centre of holes)

-the width

-the thickness and

-the teeth pitch

Clamping arrangement

Power saws are provided with clamping devices similar to those in machine vices, and the work can be gripped by using the crank  handle.

When a number of pieces of the same size are to be cut, an adjustable stop is used.

Long bars are supported, and the level maintained by the use of adjustable floor stands.

Fixing blades

The blades are mounted on frames using screws.

The teeth of the blade should point towards correct direction. (Depending on the type of machine the blade either cuts on the forward or on the return stroke).

It is necessary to follow the directions given by the manufactures as indicated in the frame.

Tension the blades using the tensioning device.

Small hole measuring gauges

. parts of a telescopic gauge

. constructional features of telescopic gauges

. parts of a small hole gauge

Telescopic gauges are popular for fine work as they are very rigid and have a better 'feel'.

Uses

Used for measuring the sizes of holes, slots and recesses.

Construction

Telescopic gauges are 'T' shaped. They consist of a pair of telescopic legs or plungers connected to a handle. The plungers are spring loaded to force them apart. After inserting the gauge in a hole or slot, it can be locked in position by turning the knurled handle. It may then be withdraw from the hole and measured with a micrometer.

Telescopic gauges are available in a set of 5 nos, to measure holes from 12.7mm to 152.4mm.

No.1 12.7mm to 19mm

No.2 19.0mm to 31.7mm

No.3 31.7mm to 53.9mm

No.4 53.9mm to 88.9mm

No.5 88.9mm to 152.4mm

Small hole gauges

Telescopic gauges are not suitable for measuring holes below 12.7mm. For measuring smaller holes and slots, small hole gauges are used.

Construction

A small hole gauge consists of tube having holes on the opposit sides at one end where hardened balls are fixed. The other end of the tube has an external thread. A screwed thimble is fixed with a threaded ttube. A plunger with a tapered end, and spring loaded, is in serted in the tube and tightened with the screwed thimble. At the end of the thimble a knurled handle is fitted. While rotating the knurled handle in a clockwise direction the plunger moves forward up, and pushes the balls out to contact the surfaces

.

A small hole gauge is an instrument used for indirect measurement, while a micrometers is usually used for measuring the sizes.

Small hole gauges are available in a set of 7 numbers to small hole gauges are available in a set of 7 numbers to measure holes from 3.2mm to 12.7mm.

No. 1 3.2mm to 3.8mm

No. 2 3.8mm to 4.5mm

No. 3 4.5mm to 5.1mm

No. 4 5.1mm to 6.4mm

No. 5 6.4mm to 8mm

No. 6 8mm to 9.6mm

No. 7 9.6mm to 12.7mm